1. Field of the Invention
The present invention relates to a method of automatically controlling a waste water purification plant wherein one or more of a number of system parameters are measured, a control parameter on the basis of the measurement results obtained and a selected control function are determined, a control action on the basis of the determined control is selected parameter, and the selected control action is implemented.
2. The Prior Art
In a prior art method of the type mentioned above, control is effected of biological waste water plants, wherein it is desired to carry out a microbial removal of the nitrogen and phosphorous containing compounds as well as organic matter.
A variety of embodiments of such biological purification plants are known, but they generally have the common feature that they comprise a nitrification tank or zone operated in aerobic conditions, a denitrification tank or zone operated in anoxic conditions and a clarification tank in which a sedimentation of active sludge is carried out and from which tank part of the active sludge is generally recycled to the nitrification and/or denitrification tank.
The above-mentioned group of purification plant types comprises two main types, viz. plants wherein recycling of completely or partially treated waste water is effected and plants comprising two treatment tanks which are alternately operated in anoxic and aerobic conditions, and in which plants no recycling of completely or partially treated waste water is effected.
In the prior art method an initial measurement is carried out of a given parameter, such as the oxygen concentration and the ammonium concentration, in the aerobic tank. On the basis of the measurement result obtained the state of the aerobic tank is identified, and subsequently the identified plant state and a preselected control criterium, e.g., maintenance of the oxygen concentration in the aerobic tank at a desired level, form the basis of a selection of a control action, e.g., in the form of a change in the oxygen supply rate.
Identification of the plant state is effected by means of a mathematical model for the relevant purification process quantitatively describing the correlation between the various measurement parameters.
The mathematical model allows i.a. control carried out using a given control criterium related to a specific measurement parameter, to be carried out on the basis of a measurement of another parameter, e.g. the oxygen set point may be controlled on the basis of a measurement of the ammonium concentration.
In another prior art method of the type described above, control is effected on the basis of measurement results for one of two or more measurement parameters. Priorities are given to the individual measurement parameters on the basis of their information value, suitability and credibility, and in normal conditions control is effected on the basis of measurement results for the measurement parameter of first priority.
If, for a period of time, it is impossible to obtain measurement results for the measurement parameter of first priority, or in case the measurement results obtained are considered erroneous and therefore have to be rejected, control is instead effected on the basis of the measurement parameter of second priority etc. This is generally known as a priority sequence of control criteria.
In a third prior art control method, control is carried out using measurements for two or more parameters simultaneously. In this control method, the measurement result of one parameter, e.g., the ammonium concentration, is used to determine the desired value (the set point) of a second parameter, e.g., the oxygen concentration. The fixed set point of the second parameter is then compared to a measurement of said parameter carried out simultaneously with the measurement of the first parameter, and on the basis of the said comparison a control action is then selected for the change of the second parameter from the measured value to the set point value. Such control method is generally known as a cascade control.
"Computer Control of an Alternating Activated Sludge Process", Kummel M. and Nielsen M. K., published at The International Symposium on Process Systems Engineering, Kyoto, August 23-27, 1982, discloses a method of controlling a biological purification plant comprising two treatment tanks which are alternately operated in anoxic and aerobic conditions, and wherein the flow pattern is changed accordingly and so that the untreated waste water is supplied to the anoxic tank, from which it is carried to the aerobic tank and therefrom further on in the plant to a clarification tank in which a sedimentation of active sludge is carried out, which sludge is subsequently recycled in the plant for introduction into the anoxic tank, and from which clarification tank the effluent is discharged.
The control is effected by means of a computer collecting the measurement results, analyzing the results on the basis of a mathematical model and implementing new control strategies.
In the prior art method, measurements of oxygen, ammonium and nitrate are carried out using suitable sensors, the control parameters used being the oxygen supply rate and the nitrification and denitrification period ratio.
In the prior art method, the ammonium and nitrate concentration methods are used continuously to determine the corresponding optimum oxygen concentration (the set point) during the nitrification and denitrification processes, respectively.
Furthermore the nitrification and denitrification period ratio is controlled relative to the ammonium content of the untreated waste water, i.e., such that the nitrification period is prolonged when the ammonium load is high and shortened when the ammonium load is low, and vice versa for the denitrification period.
EP-A-0,446,036 discloses an apparatus for controlling a system, e.g. a waste water purification plant, the apparatus:comprising 1) a number of measuring units, 2) means for analysing measurement data in order to select a characteristic data set, 3) means for analysing the characteristic data set in order to identify a possible operation problem, 4) means for analysing the operation problem in order to find a strategy for resolution of the problem, and 5) means for controlling the system on the basis of the strategy.